The present invention relates to a resin composition for a porous film or sheet, a porous film or sheet, and a method of producing the same.
Porous synthetic resin films having minute pores which do not pass water drops therethrough but allow gas such as vapor to pass therethrough, have recently been come to use in the clothing field, the medical field, etc. Such films have a function of preventing from becoming stuffy and passing through no liquid. They are used for throwaway diapers, raincoats, throwaway sheets, etc.
One of the typical methods of producing such a porous film having air permeability (breathability) is a method comprising the steps of mixing an inorganic filler such as calcium carbonate, talc and clay with a synthetic resin, forming a film therefrom and stretching the thus-obtained film to form fine cracks on the film.
A porous film or sheet obtained in the said method has a problem in which physical properties such as anisotropy, the balance between tensile strengths in the machine direction and in the transverse direction, and the surface strength are inferior.
To solve these problems, a method of producing a porous film or sheet comprising extruding a composition containing a specific amount of linear polyethylene, branched low-density polyethylene, radical generator and filler, and stretching the obtained film or sheet (U.S. Pat. No. 5,015,521).
However, a porous film or sheet which has a more excellent tear strength in addition to a good balance between tensile strengths in the machine direction and in the transverse direction and an excellent surface strength, and in which the unevenness of stretching and the nonuniformity in film thickness are ameliorated, is further demanded.
As a result of various studies undertaken by the present inventors in order to meet such demand, it has been found that by forming a film or sheet by an inflation method from a composition containing as a plasticizer a compound having an ester linkage or an amide linkage in a molecule and having a molecular weight of not less than 100, a boiling point of not lower than 200.degree. C. and a melting point of not higher than 100.degree. C. under an ordinary pressure, and a radical generator, and uniaxially stretching the resultant film or sheet in the machine direction, the obtained porous film or sheet has more excellent tear strength and surface strength, and scarcely suffers from the unevenness of stretching and the nonuniformity in film thickness. The present invention has been achieved on the basis of this finding.